extension Calendar {
    /**
     Returns a tuple containing the start and end dates for the week that the
     specified date falls in.
     */
    func weekDatesForDate(date: NSDate) -> (start: NSDate, end: NSDate) {
        var interval: TimeInterval = 0
        var start: NSDate?
        range(of: .weekOfYear, start: &start, interval: &interval, for: date as Date)
        let end = start!.addingTimeInterval(interval)

        return (start!, end)
    }
}

extension NSCalendar {
    /**
     Returns a tuple containing the start and end dates for the week that the
     specified date falls in.
     */
    func weekDatesForDate(date: NSDate) -> (start: NSDate, end: NSDate) {
        var interval: TimeInterval = 0
        var start: NSDate?
        range(of: .weekOfYear, start: &start, interval: &interval, for: date as Date)
        let end = start!.addingTimeInterval(interval)

        return (start!, end)
    }
}